So-called two-part containers, such as carboys, have in recent years been constituted by an inner container or lining of synthetic-resin thermoplastic foil, or a so-called soft thermoplast, such as high pressure polyethylene, and an outer supporting structure or cardboard or other material.
It is common practice to form the inlet and outlet fitting or spigot of the inner container by injection molding as a tubular, relatively hard synthetic-resin body having a foot underlying the foil surrounding an opening formed in the inner container. The fitting may be provided with formations adapted to receive a threaded cap by which the carboy is closed in a fluid-tight manner.
A relatively thick-walled injection molded fitting is fitted through the opening and can be welded around to the thin-walled material of the inner container. The welding should be fluid-tight and sufficiently strong to prevent damage during transport and handling of the receptacle.
It is known to weld the foil of the thin-wall container, consisting of a plastified synthetic resin and produced by blow-molding or vacuum-forming, to the preformed injection-molded thick-walled fitting by the use of ultrasonic welding techniques (see U.S. Pat. No. 3,947,307). In the device described therein, the fitting is provided with an annular flange or foot which underlies the foil surrounding the opening of the thin-walled container and is bonded thereto by ultrasonic energy between an ultrasonic head or sonotrode and an anvil which retains the rear wall of the foil container and forms a support for the foot of the fitting as the Sonotrode or ultrasonic head presses the overlying foil layer of the container against the flange of the fitting.
The injection-molded fitting, although preformed, is constituted of the same material as the synthetic-resin foil constituting the container, the annular head of the Sonotrode bearing along an annular zone surrounding the opening in the thin-wall container and against the flange underlying the same and unitary with the fitting. The anvil, as noted, supports the fitting through the opposite wall of the container. The annular foot or flange of the fitting has a thickness equal at least to ten times the wall thickness of the foil constituting the container and the anvil is specially shaped to provide the annular support which is required.
More particularly, as described in the aforementioned patent, the anvil comprises an upper member and a lower member forming the annular support surface for the weldng zone at the annular foot or flange of the fitting. The lower member is spaced from the upper member by a narrow gap and the upper member is centered with respect to the lower member by a wobbling or floating ball which is recessed in each of the two members. The purpose of the ball is to distribute the pressure applied during the ultrasonic welding action uniformly upon the flange of the fitting and the portion of the foil surrounding the opening of the container when the head is pressed toward the anvil. The intent thereby is to assure a uniform and fluid-tight welding at the interface of the flange of the fitting and the zone surrounding the opening of the foil container.
Although this device has been found to be highly effective, it has also been found to cause some problems.
Firstly, remote welding during the application of ultrasonic radiation and pressure cannot be prevented and such welding at locations other than those desired is a significant disadvantage. Moreover, there is a problem with bonding of the lower or rear foil wall of the container to the foot of the tubular fitting or spigot.
It has also been found that the ultrasonic vibrations are not uniformly applied to the relatively thick foot of the fitting or spigot and that such radiation frequently detrimentally affects the foil wall opposite the spigot notwithstanding efforts to limit the ultrasonic effects to the welding interface.
In practice it is found that the system can be operated only with relatively low ultrasonic energy if some of the other disadvantages enumerated above are to be avoided or limited.
Even the arrangement of the upper part of the anvil via the ball of the lower part of the anvil has a disadvantage because the ball does not always allow complete self-leveling and pressure equalization as rapidly as is desired, particularly when the welding process is held short. Such compensation is required since the foil thicknesses between the head or Sonotrode and the anvil frequently vary within fairly wide tolerances. The welding process is thus terminated before the ball system has had an opportunity to fully compensate for variations in the foil thickness and as a result a reduced weld quality is obtained and leakage may occur.
The latter disadvantage can only be limited at low ultrasonic energies with low applied pressures and corresponding long welding times are used. Since the welding time is increased to compensate for the aforementioned drawback, high energy consumption over long periods, is the undesired result.